Yes, it can feel right and good to support gay marriage, especially outside a religous context. The Catholics who support it do so with the best of intentions.
I see two problems
- historically, people have hated the sinner. Catholics are right to acknowledge the injustice of this hate, but may be confused in their response
- many Catholics no longer see the same sex act, or sex outside of marriage as a sin. This is where they depart from the Vatican and LDS postion
As an ex-catholic I would say the LDS put significant effort into educating the flock on their beliefs and teachings. This will lead to more unity on controversal topics such as this